The invention relates to an in-line adsorber system for an internal combustion engine, capable of meeting the California ultra-low emission vehicle (ULEV) standard, using a modified adsorber construction.
Internal combustion engines emit large amounts of unburned hydrocarbons during the cold start of an engine due to the rich fuel mixture used in such engines, and the necessarily incomplete combustion at start-up. This emission of unburned hydrocarbons continues until the main catalyst reaches its "light-off" temperature, at which point the catalyst begins to convert the hydrocarbons to harmless gases. The typical catalytic light-off time for most internal combustion engine systems is around 50 to 120 seconds, during which time significant amounts of hydrocarbons are emitted into the atmosphere. The actual catalytic light-off time for any system will depend on the position of the catalyst relative to the engine, as well as the noble metal loading. The temperature of the catalyst is elevated by contacting it with the high temperature exhaust gases from the engine, and continuous contact with those gases along with the exothermic nature of the oxidation reactions occurring at the catalyst combine to maintain the catalyst at an elevated temperature.
While catalytic converters are well known for reducing oxides of nitrogen (NOx), and oxidizing hydrocarbons and carbon monoxide from automobile exhaust, these reactions typically take place after the catalyst has attained its light-off temperature. That is, at temperatures generally in the range of 200-350.degree. C. depending on such factors as the noble metal loading and aging of the catalyst. Typically however, seventy to eighty percent of hydrocarbon emissions from automotive vehicles are emitted during about the first minute of engine operation, during which time in most systems, the main catalytic converter has not attained light-off and is therefore, not active. As a result, during cold-start large amounts of hydrocarbons may be discharged into the atmosphere if additional measures are not taken. The problem is made worse by the fact that the engine requires rich fuel-air ratio to operate during cold-start thus, increasing even further the amount of unburned hydrocarbons discharged. Therefore, to increase the effectiveness of automotive emission control systems during cold start, and more importantly, the ULEV standards require that, the amount of hydrocarbons discharged into the atmosphere during cold-start must be kept to extremely low levels.
Various schemes have been proposed for meeting the stringent ULEV standards during cold start including through the use of electrically heated catalysts (EHCs) to reduce the light-off time of the main catalyst. Another suggested scheme includes the use of molecular sieve structures (hydrocarbon adsorbers) to adsorb and hold significant amounts of hydrocarbons until the converter has attained its light-off temperature. Still, other schemes have been suggested involving a combination of electrically heated catalysts and adsorbers. Recently, improved in-line and by-pass exhaust control systems respectively have been disclosed in co-pending, co-assigned U.S. application Ser. No. 08/234,680 and 08/259,459 (both herein incorporated by reference), using bi-metallic valves to control exhaust gas flow during cold-start. In the former, a hollow molecular sieve structure having a bi-metallic valve is used to achieve the ULEV standards. Co-pending, co-assigned U.S. application Ser. No. 08/284,356 (Guile), filed concurrently herewith and herein incorporated by reference, discloses a by-pass adsorber system wherein flow patterns from a secondary air source are used to direct exhaust gas flow to and away from the adsorber during cold-start.
There continues to be a need for, and accordingly, it is the object of the present invention, to provide an even simpler and more improved engine exhaust systems capable of meeting the strict California ULEV standards.